Llanelli, who had earlier lost 22-13 in Cardiff, stopped the Blue and Blacks from completing the double by edging them out 13-11 at Stradey Park in the Under-21's Premier Division.
Nick Macleod, who recently made his senior debut against Cambridge University, kicked the only points of the first-half with a penalty that gave Cardiff the lead at 3-0.
Llanelli came back into the match when acting captain Aled Gravell scored a try and he was soon followed over the line by prop forward Phil John with fly half Phil Fisher kicking a penalty goal.
At 13-3, Cardiff retaliated with a try by wing Geraint Williams and a second penalty by Macleod, but the latter's conversion attempt that would have provided the equalising points went wide.
Llanelli turned out their Academy 17-year-olds against Bath at Stradey Park and pulled off an excellent 18-0 win.
Wing Dafydd Evans ran the first try in from 50 metres and full back Ricky Williams placed a 30-metre penalty to lead 8-0 at the break.
In the second spell, centre Aled Thomas raced in for the second Scarlet try and a smart pass by fly half Roger Davies put Williams away for the final score.
Earlier, Cardiff's under-21's had scored a resounding 41-15 win at Swansea, to avenge their 31-29 loss at home in the first meeting. It was one of the best Cardiff performances that included tries by full back Owain Ashman, scrum half Scott Heddington, lock Luke Tait, wings Elgan Jones and Geraint Williams and centre Liam Roberts.
Nick Macleod placed four conversions and a penalty, but Swansea had led 15-13 at half-time and then lost lock Bill Brereton with a broken ankle. Prop Steve Tremlett and Brereton scored their tries with fly half Gareth James adding a penalty and conversion.
Wales U20 duo Leon Brown and Lloyd Lewis face a busy couple of weeks ahead. Not only will they have a Junior World Championship to take part in, they are both sitting A Level exams during the competition.
Wales U20 face a tough group in the Junior World Championship with France, England and Japan lying in wait but Attack coach Jason Strange and back rower Tom Phillips are looking forward to the campaign ahead.
Angus O'Brien has quickly settled into the Wales Sevens squad after solid performances in Hong Kong and Japan. With two rounds remaining in Glasgow and Twickenham on the World Series, WRU TV's Graeme Gillespie chats to the rising star.
Wales winger Luke Morgan has established himself as one of the deadliest finishers on the Sevens World Series. WRU TV's Graeme Gillespie catches up with him on the eve of this weekend's round in Glasgow.
All roads lead to Cardiff on Sunday for the SSE SWALEC Finals Day at the Millennium Stadium. Cambrian Welfare tackle Ystradgynlais in the Bowl final, Ystrad Rhondda play Newcastle Emlyn in the Plate final while defending champions Pontypridd face Bridgend in the Cup final.
WRU Chief Executive Roger Lewis, Deputy Minister for Culture, Sport and Tourism Ken Skates and WRU Chairman Gareth Davies earlier this week launched a new competition in conjunction with the WRU unveiling its archive room.
Welsh players and management joined Deputy Minister for Sport Ken Skates at the Pierhead Building, Cardiff Bay earlier this week to reveal the Union's pioneering school-club hub initiative has been so successful the project will now almost double the scale of the original project.
The Scarlets, Newport Gwent Dragons, Cardiff Blues and the Ospreys look ahead to Judgement Day III which sees the Blues take on the Ospreys (2.30) and the Dragons tackle the Scarlets (4.45pm) at the Millennium Stadium tomorrow afternoon.
Ystradgynlais and Ynysddu played out a titanic struggle at Taffs Well in the SSE SWALEC Bowl semi-final. At the end of the game neither team could be separated after a 15-15 stalemate but Ystradgynlais go through on try count.